Heat exchangers which are used for automobile air conditioner usually have a complicated structure in which in order to widen a surface area of the heat exchange as far as possible, aluminum fins are kept at narrow intervals, and furthermore, an aluminum tube for feeding a coolant into these fins is arranged complicatedly. The moisture in air attaches as condensed water to the fin surfaces at the time of operation of the air conditioner; however, on that occasion, the condensed water becomes a water drop in a substantially hemispherical shape on the fin surfaces with poor wettability or exists in a bridge form between the fins, thereby disturbing a smooth flow of exhaustion and increasing the ventilation resistance. If the wettability of the fin surfaces is poor, the heat exchange efficiency is lowered.
Furthermore, in general, aluminum or an alloy thereof constituting an aluminum fin and an aluminum tube (hereinafter referred to as “aluminum fin and so on”) is originally excellent in rust preventing properties; however, if the condensed water stays on the fin surface over a long period of time, an oxygen concentration cell is formed, or contaminant components in the air gradually attach thereto and are concentrated, whereby a hydration reaction or a corrosion reaction is accelerated. This corrosion product is accumulated on the fin surface, whereby not only it impairs heat exchange characteristics, but it is scattered as a white fine powder by an air blower.
Then, in order to improve these problems, for example, a surface treatment method in which after cleaning an aluminum material-made heat exchanger with an acid, the heat exchanger is dipped in a zirconium based chemical conversion treatment liquid to achieve a zirconium chemical conversion treatment and then dipped in a hydrophilic treatment liquid composed of a mixture of modified polyvinyl alcohol, a phosphorus compound salt, a boron compound salt, a hydrophilic organic compound, a crosslinking agent, and the like, to achieve a hydrophilic treatment, thereby imparting satisfactory hydrophilicity and deodorizing properties onto the aluminum surface; and the like are proposed (see Patent Document 1).
On the other hand, in an aluminum material-made heat exchanger to be used for automobile air conditioner, after assembling a lot of aluminum fins and aluminum tubes, the aluminum fin and the aluminum fin, or the aluminum fin and the aluminum tube, are joined; however, since a firm and minute oxide film is formed on the surface of aluminum, joining other than a mechanical joining process, such as brazing, soldering, etc., cannot be simply achieved, and the VB process (vacuum brazing process) of achieving brazing in vacuo was chiefly carried out.
However, in recent years, a halogen based flux was developed as a measure for effectively removing or breaking the oxide film, and for such reasons as easiness of control of brazing, inexpensiveness of a furnace, inexpensiveness of brazing processing, etc., a flux brazing process represented by the NB process for performing brazing in a nitrogen gas has been adopted.
This NB process is a process of assembling the aluminum fin and so on and then brazing the aluminum fin and so on in a nitrogen gas by using a flux such as KAlF4, K2AlF5, etc., and the NB process has also been applied to the fabrication of a heat exchanger in an automobile air condition.
Nevertheless, in the NB heat exchanger fabricated by this NB process, the flux inevitably remains on the aluminum surface, and therefore, there are involved problems which are inherent to the NB heat exchanger such that the surface state becomes heterogeneous; that a uniform surface treatment such as a chemical conversion treatment, a hydrophilic treatment, etc. cannot be achieved; and that corrosion resistance, adhesion, and the like become insufficient.
For that reason, so far, in the surface treatment of the NB heat exchanger, (1) a flux removal step, (2) a chemical conversion treatment step (rust preventing step), and (3) a hydrophilic treatment step are successively performed; however, there is involved such a problem that halogen based wastewater is produced in the flux removal step. Also, in the flux removal step, in order to enhance chemical conversion properties, etching with an acid or an alkali is performed; however, since only the flux cannot be removed, there is involved such a problem that excessive etching on the aluminum fin occurs, so that a uniform chemical conversion treatment cannot be achieved.
Furthermore, in view of a problem of safety to the human body, a chemical conversion treatment which is free from blending of Cr6+ is necessary; however, in that case, the corrosion resistance is insufficient. Moreover, a reduction of the number of processes was required, too.
In order to cope with these problems, as a method of surface treating the NB heat exchanger, for example, a surface treatment method in which an NB heat exchanger is dipped in a zirconium based chemical conversion treatment liquid to achieve a zirconium chemical conversion treatment and then dipped in a hydrophilic treatment liquid composed of a mixture of polyvinyl alcohol, a polyoxyalkylene-modified polyvinyl alcohol, an inorganic crosslinking agent, a guanidine compound, and the like, to achieve a hydrophilic treatment, thereby imparting a deodorizing effect in addition to satisfactory anti-corrosive and hydrophilic effects; and the like are proposed (see Patent Document 2).
On the other hand, in particular, as a hydrophilic treatment agent capable of forming a film which is excellent in hydrophilicity and corrosion resistance and also excellent in prevention of frost formation on a heat exchanger fin material of an outdoor unit, a hydrophilic treatment agent for heat exchanger fin material containing (a) a silicate of a metal selected from alkali metals and alkaline earth metals, and preferably lithium silicate, (b) polyvinyl alcohol, and (c) an acrylic resin having a weight average molecular weight falling within the range of from 3,000 to 300,000 and a resin acid value of 400 mg-KOH/g or more, is disclosed; and furthermore, a hydrophilic treatment method for a heat exchanger aluminum fin material, in which this hydrophilic treatment agent is applied onto the surface of an aluminum fin material and baked, thereby forming a film having a dry film thickness of from 0.2 to 5 μm is proposed (see Patent Document 3).